Although the concept of Automatic Gain Control (AGC) in radio signal reception is well understood, automatic gain control of Time Division multiplexed Multiple Access (TDMA) signals presents new challenges to the land-mobile industry.
In wideband TDMA systems, such as the cellular system proposed for use in Europe, an RF channel is shared (time-division-multiplexed) among numerous subscribers attempting to access the radio system in certain ones of various time-division-multiplexed time slots. The time slots are arranged into periodically repeating frames. Thus, a radio communication of interest may be periodically discontinuous--interleaved with unrelated signals transmitted in other time slots. The unrelated signals (of widely varying strength) must not influence the gain control of the signals of interest. A formidable challenge then is to provide Automatic Gain Control of these periodically discontinuous TDMA signals.
The challenge is further enhanced by attempting to provide digital AGC in inexpensive receivers--those having limited dynamic range. Since these signals may vary by as much as 100 dB in the land-mobile environment, but modest 8-bit Analog-to-Digital converters (A/D) for digital signal processing are limited to 48 dB dynamic range, techniques must be developed for controlling the gain of the signal to keep it within the limited dynamic range of the receiver. The challenge then is to handle a 100 dB discontinuous signal with a 48 dB device; otherwise, prohibitively expensive A/Ds with greater dynamic range must be utilized.
Another challenge for gain control is introduced by the digital nature of these TDMA transmission systems. Gaussian Minimum Shift Keying (GMSK) modulates the quadrature phases of the signal such that the power of the received signal is more difficult to measure, and neither of the quadrature phases, taken alone, is proportional to the received signal power.
This invention takes as its object to overcome these challenges and realize certain advantages, presented below.